Over the past few decades, demand for semiconductor devices has grown rapidly. Semiconductor manufacturers are often pressured into improvements in end-product quality, speed and performance, as well as improvements in manufacturing process quality, speed and performance. Machine vision has proven to be a very essential part of improving the productivity and quality of semiconductor production. There is a consistent drive for faster and more accurate machine vision systems for ever-higher semiconductor yields. Many high-density semiconductor packaging inspection applications require three-dimensional measurement capability. Correspondingly, the technical field of three-dimensional measurement and inspection for semiconductor devices, such as semiconductor wafers or substrates on final packaged products, has seen rapid growth.
Many commercial systems use triangulation-based principles for three-dimensional measurement such as that disclosed in U.S. Pat. No. 6,064,756 entitled “Apparatus for Three Dimensional Inspection of Electronic Components.” It describes a three dimensional inspection apparatus for a ball array device which is positioned in a fixed optical system. A first camera is disposed in a fixed focus position relative to the ball array device for taking a first image of the ball array device to obtain a characteristic circular doughnut shape image from a ball. A second camera is disposed in a fixed focus position relative to the ball array device for taking a second image of the ball array device to obtain a top surface image of the ball. A processor applies triangulation calculations on related measurements of the first image and the second image to calculate a three dimensional position of the ball with reference to a pre-calculated calibration plane.
Other triangulation-based systems may utilize focused laser or fringe pattern projection. In such triangulation approaches, a laser or other structural pattern projects light or patterns onto an object surface and a sensor is inclined with respect to the incident light or pattern. There is a drift in a position of the light or pattern detected by the sensor when the height varies. Height information can be measured from the drift position detected on the sensor. However, the triangulation setup is vulnerable to occlusion and shadows. Moreover, due to the measurement range required by semiconductor packages like Ball Grid Array (BGA) packages, its pixel/spot resolution is typically poorer than 10 μm. As such, there is insufficient optical resolution due to the required measurement range. It would be necessary to enhance the optical resolution to meet the high-accuracy demands required by the latest packaging technology.
Confocal optical devices make use of the principle that an output signal is at a peak (in intensity or contrast) at a focal plane of the confocal optical device. It utilizes a diffraction-limited spot with a large Numerical Aperture (N.A.), and thus it is capable of submicron optical resolution. It needs two scans, for instance a rotating Nipkow disk or other scanning method to scan a horizontal XY plane and vertical Z movement to scan a target depth of field given a very small depth of focus of the confocal optical system.
US Patent Publication Number 2010/0296106 A1 entitled “Chromatic Confocal Sensor” discloses a confocal optical system comprising a substrate having thereon a multiphoton curable photoreactive composition, a light source that emits a light beam comprising a plurality of wavelengths onto at least one region of the composition on the substrate, and a detector that detects a portion of light reflected from the composition to obtain a location signal with respect to the substrate, wherein the location signal is based at least on a wavelength of the reflected light. In this way, a height of an object surface can be determined.
Unfortunately, the aforesaid chromatic confocal sensor can only inspect one single dot on the object surface at any one time. Therefore, there is a need to move the sensor two-dimensionally on a horizontal plane to scan the whole object surface. As such, measurement of the whole of a three-dimensional surface is very time consuming and far from the high throughput required by the semiconductor industry.
Some slit scanning systems make use of the dispersive properties of diffractive lenses by which the image planes of a slit are wavelength dependent and are uniformly distributed along a longitudinal direction. Wavelength-coded light rays with variable foci are then imaged onto a measured sample through a coupling lens and a microscope objective lens. Specifically, US Patent Publication number 2010/0188742 entitled “Slit-Scan Multi-Wavelength Confocal Lens Module and Slit-Scan Microscopic System and Method Using the Same” discloses a slit-scan multi-wavelength confocal system which utilizes at least two lenses having chromatic aberration for splitting a broadband light into continuously linear spectral lights having different focal lengths respectively.
The aforesaid slit-scan confocal systems make use of conventional circular lenses as a microscope objective lens. Yet, owing to manufacturing difficulty, the Field of View (FOV) of commercial microscope objective lenses is very limited, especially those with large N.A. Therefore the inspection speed is also limited. Furthermore, the N.A. of a circular objective lens is symmetrical and thus any out-of-focus light emerging from the slit which is projected on the object cannot be suppressed as in a conventional spot-scan confocal system. Therefore, this will greatly affect the measurement accuracy.